The present invention relates to heat treatment furnaces and more particularly, to an improved aluminum solution-type heat treatment furnace and method of constructing the same.
As is well known, many steel and aluminum fabricated parts must be heat treated to modify the physical properties (i.e., hardness of the parts) to suit their intended structural use. With specific reference to the heat treatment of aluminum fabricated parts, the heat treatment process contemplates the placement of the parts into an aluminum solution-type heat treatment furnace wherein the temperature of the aluminum parts is raised by convectional heat transfer to a predetermined temperature within a specific time interval, and subsequently quenched within a water or water/glycol bath to yield the desired physical properties to the aluminum part.
Conventional aluminum solution-type heat treatment furnaces have heretofore been constructed having an external furnace housing, the interior surface of which is lined with a plurality of rigid ceramic blocks, arranged in a brick wall-like orientation to form an insulating bearer and retard heat transfer through the housing. Typically, the rigid ceramic blocks mount along their inner surface, plural calrod or ribbon type resistance heating elements which are adapted to raise the temperatures of the air within the furnace to predetermined levels. A heat treatment chamber is defined within the interior of the furnace and is typically separated from direct communication with the heating elements. The top of the heating chamber typically communicates through plural opening or vents with an air plenum, while the sides of the chamber communicate with the heating elements through one or more damper openings.
In operation, air is supplied to the air plenum by circulating fans and is forced downward through the vents into the heating chamber and across the fabricated parts disposed therein to effectuate convectional heat transfer from the air to the fabricated parts. The air flow subsequently exits the heat treatment chamber through the chamber dampers and is pulled across the plural heating elements to be raised in temperature and subsequently recirculated to the plural fans. Usually, the plenum vents and chamber dampers may be manually adjusted from the interior of the heat treatment chamber to control the air flow through the chamber and, hence, the operating temperature of the furnace. Once the prefabricated part has been raised to a sufficient temperature within a predetermined time, the part is transported directly into a quenching bath to effectuate a proper heat treatment of the prefabricated part. Although such prior art heat treatment furnaces have proven generally suitable for their intended use, they possess inherent deficiencies which detract from their overall effectiveness and efficiency.
A major deficiency of the prior art heat treatment furnaces has been their propensity to provide a random air flow path through the heat treatment chamber which results in temperature gradients existing within the heat treatment chamber. Such non-uniform temperatures within the heat treatment chamber often cause variations in the resultant heat treatment properties of the prefabricated parts, and in instances, fail to allow the furnace to be certified for the desired heat treatment operation.
An additional deficiency of the prior art heat treatment furnaces has been their requirement of adjusting the plenum vents and chamber dampers exclusively from the interior of the heat treatment chamber. As such, when it was necessary to adjust the vents and dampers in an attempt to effectuate more uniform temperatures within the heating chamber, the prior art heat treatment furnaces mandated a complete shut-down and cooling of the chamber with a technician subsequently entering within the interior of the chamber and manually adjusting the vents on a trial and error basis. Subsequently to vent and damper adjustment, the furnace had to be heated back to operating temperatures and suitable inspections performed to insure the temperature gradients existing within the heat treatment chamber were within suitable tolerances. Oftentimes, such trial and error adjustment or certification of the furnace extended for weeks and even months, which necessarily detracted from the overall cost effectiveness of the heat treatment process.
Further, the use of the rigid ceramic block or refractory brick and bendover rod heating assemblies of the prior art furnace has proven to be extremely prohibitive during manufacture as well as during operation, in that substantial heat energy is dissipated from the blocks during the quenching process which necessitates rather long recovery times for the furnace. Additionally, the prior art calrod heating elements generate substantial quantities of radiant heat energy during operation, which is a detriment to aluminum heat treatment applications and, further, due to their relatively large mass, have proven generally difficult to precisely control to effectuate a proper temperature within the heat treatment chamber.
Although the heat loss deficiencies of the prior art ceramic block design have been recognized in the art with the recent industry introduction of ceramic fiber insulation having improved insulating characteristics, the use of such ceramic fiber insulation has been limited exclusively to calrod heating element assemblies. Hence, the radiation and temperature control deficiencies of the prior art have remained substantially unaddressed. In addition, although very recent attempts have been made to utilize such ceramic fiber insulation with ribbon-type heating elements which do not possess the radiant and temperature control deficiencieis of calrod elements, such attempts have failed primarily due to the inability of the ceramic fiber insulation to possess sufficient structural integrity to rigidly support the required tensile mounting forces with ribbon-type heating elements.
Hence, there exists a substantial need in the art for a heat treatment furnace which provides uniform air flow throughout the heat treatment chamber, permits adjustment of the plenum vents and chamber dampers during the heat treatment operation, and incorporates a ceramic fiber insulation/ribbon-type heating element assembly and method of construction to improve the temperature control and the quench recovery time of the heat treatment furnace.